ontinuous, though it is necessarily not mutual, and hence
there is no ecotone during the earlier stages. The significant feature
of continuous invasion is that an outpost may be repeatedly reinforced,
permitting rapid aggregation and ecesis, and the production of new
centers from which the species may be extended over a wide area.
Contrasted with continuous invasion is intermittent or periodic movement
into distant regions, but this is rarely concerned in succession. When
the movement of invaders into a community is so great that the original
occupants are driven out, the invasion is _complete_.

A topographic feature or a physical or a biological agency that
restricts or prevents invasions is a barrier. Topographic features are
usually permanent and produce permanent barriers. Biological ones are
often temporary and exist for a few years or even a single season.
Temporary barriers are often recurrent, however. Barriers are complete
or incomplete with respect to the thoroughness of their action. They may
affect invasion either by limiting migration or by preventing ecesis.

Biological barriers comprise plant communities, man and animals, and
parasitic plants. The limiting effect of a plant community is exhibited
in two ways. In the first place, an association acts as a barrier to the
ecesis of species invading it from associations of another type, on
account of the physical differences of the habitats. Whether such a
barrier be complete or partial will depend upon the relative unlikeness
of the two areas. Shade plants are unable to invade a prairie, though
the species of open thickets or woodland may do so to a certain degree.
Closed communities (one in which all the soil is occupied) likewise
exert a marked influence in decreasing invasion by reason of the intense
and successful competition which all invaders must meet. Closed
associations usually act as complete barriers, while more open ones
restrict invasion in direct proportion to the degree of occupation. To
this fact may be traced the fundamental law of succession (the law by
which one type of community or formation is succeeded by another) that
the number of stages is determined largely by the increasing difficulty
of invasion as the area becomes stabilized. Man and animals affect
invasion by the destruction of germules. Both in bare areas and in seral
stages the action of rodents and birds is often decisive to the extent
of altering the whole course of developm